Rotary electric furnaces have been used since at least the 1930's for the heating and/or fusion of minerals, such as quartz, and other materials having a high melting point. In such furnaces, the material being heated and/or melted is pressed against the walls of the furnace by centrifugal force and may be used in lieu of a refractory lining to insulate the furnace from the heat produced by the electrical power source.
Such furnaces normally include a cylindrical shell, the outer surface of which is engaged by a drive means to effect rotation of the shell. The cylindrical shell is normally closed by ends which have an axial opening to permit the introduction of an electrode. Over the years, both flat, frusto-conical, and conical ends have been employed in such furnaces, the later because they tend to more closely approximate the inherent cocoon (prolate spheroid) shape of the ingot formed inside the rotary furnace, thereby reducing the amount of raw material needed to fill the furnace for the production of a given size ingot. Such furnaces are disclosed, for example, in British Patent No. 330,943 to Heraeus; H. George, "Electric Furnaces With Carbon Radiator, " Transactions of the American Institute Of Electrical Engineers, Vol. 54, pp. 1195-1199 (1935); U.S. Pat. No. 2,104,555 to Cousteix; U.S. Pat. No. 3,937,866 to Sunnen et al.; and U.S. Pat. No. 4,217,462 to Rawles et al.
More recently, applicant's assignee has employed rotary electric arc furnaces for silica fusion which use ends having a reduced diameter frusto-conical portion adjacent to the cylindrical shell and a removable cylindrical cover attached to the smaller end of the frusto-conical portion. In this configuration, the frusto-conical portion is designed to conform to the shape of the ingot to be produced in the furnace and to reduce the amount of raw material needed to fill the furnace. The cylindrical cover is designed to reduce heat distortion and warpage of the furnace ends that tends to occur close to the axis of the furnace, where the electrodes are inserted and where there is little, if any, self-insulating raw material to protect that portion of the furnace from the heat generated by the electrodes. Normally, this cylindrical cover is lined with a ring-shaped refractory insert to further reduce heat damage to the ends of the furnace.
However, the frusto-conical ends of such furnaces are difficult to fabricate and rather expensive to produce. In addition, furnaces employing the frusto-conical design experience a lack of balance during rotation, resulting in an increased wear and tear of parts of the rotary furnace.